This invention relates to a clamp assembly for use with power cables, and more particularly with a clamp assembly for connecting the metal armor, screen or shielding of a power cable to a metal bridging element which spans the joint and is connected to the metal armor, screen or shielding of another power cable of the joint. This invention also relates to a method of protecting a joint between power cables using a joint case having a continuous metal element, a dimensionally-recoverable polymeric sleeve and said clamp assembly.
Electric power cables are of two general types, paper insulated and polymer insulated. Paper insulated cable comprises an inner core containing, usually, three or four current carrying conductors. The conductors are insulated from each other and are surrounded by an insulation comprising oil impregnated paper. A metal sheath, generally of lead or aluminum, surrounds the insulation to protect the core from moisture. Surrounding the metal sheath is a layer of armor which provides longitudinal strength and mechanically protects the metal sheath. The armor is made of metal, generally steel or aluminum and can be in the form of wires, sheet or tape. The armor and metal sheath may be required to carry significant earth fault currents. The armor is provided with additional outer protection, which typically is a bitumen/hessian combustion, but which can also be a polymeric layer such as polyvinyl chloride. Where bitumen/hessian are used as outer protection, this layer does not effectively prevent water, after long periods in service, from penetrating through to the armor layer and then wicking down the individual metal components (or layers). The oil impregnated paper insulation is particularly sensitive to water and where the lead or aluminum sheath is removed, such as when a joint between power cables is made, steps must be taken to prevent water coming into contact with the insulation and/or conductors of the core.
A polymer insulated cable consists of one or more insulated conductors. The conductors may be solid metal or of stranded construction and are typically of copper or aluminum. The insulating polymer is typically cross-linked polyethylene, polyvinyl chloride or ethylenepropylene rubber and is generally applied by extrusion. Individual insulated conductors are known as cores. Numbers or cores are layed together to form a cable. Most commonly, polymer insulated power cables contain one, three or four cores. For practical purposes, the cores of low voltage power cables require additional protection which usually consists of one or more outer plastic sheaths applied by extrusion and metal wires of tapes wrapped around the layered cores and directly beneath the outer protective jacket. Depending on their size, type and the construction of the cable, these metal wires or tapes are known as screening or armoring. In some cases, these metal components are embedded in a compound to prevent the progress of moisture along individual strands.
Higher voltage polymer insulated power cables are of essentially similar construction but with minor, but nonetheless, significant differences. The core insulation is typically thicker, of high quality and each core is surrounded by its own screen. These core screens vary widely, depending on manufacturer and user requirements and may consist of the following used separately or in combination: copper wires or tapes, carbon black impregnated fabric tapes, extruded low resistance plastic tapes, conductive coatings applied to the surface of the core insulation by extrusion, dispersion coating and the like.
Various combinations of armor, screen, insulation etc. are possible and a variety of power cables are used. Power cables are connected together making a joint. A joint may consist of one cable jointed to at least one other power cable. In making the joint, the various layers of insulation, metal sheath, armor, shielding, etc. are removed from the ends of conductors to be joined. The area of the joint must be protected from the environment, particularly from moisture ingress after the joint has been made. It is also necessary to interconnect the metal armor, screen or shielding of the power cables of the joint.
In the past, the joint area has been protected by placing a joint case, generally a metal box, around the joint. The metal box is sealed and electrically connected to the metal sheath of the cable and the metal armor, screen or shielding using solder. The joint case is filled with a potting composition, usually bitumen or certain curable resin systems which are known in the art. The metal joint case is usually of lead, steel, tinned copper or cast iron and provides a fault current path across the joint as well as protection from moisture ingress. Joint cases of molded plastic can also be used. In this case, a metal braid is soldered to the metal armor, screen or shielding of one cable of the joint, extends across the joint and is connected to the other cable to provide a fault current path across the joint.
A new joint case for power cables is described in U.S. Pat. No. 4,282,397 of Siedenburg and Fritsche 5. This splice case comprises a hollow relatively rigid cylindrical shell having a longitudinal split along its length so that it can be wrapped around an already formed joint. The shell is preferably of metal and has a polymer sheet bonded to its external surface. The shell has tapered end regions formed of prongs or fingers. When the case is used to protect a joint between power cables, the tapered ends of the shell are soldered to the lead or aluminum sheath of the power cable. The metal armor, screen or shielding is also soldered to the metal sheath, preferably at the same points so that only one solder joint is required at each end. A fault current path is thus provided across the joint by the metal shell of the joint case as it is electrically connected to the jointed cables. The assembly is environmentally sealed with a dimensionally recoverable sleeve extending over the joint case or by two shorter sleeves over the tapered end regions.
In the methods described above, a metal braid and/or the joint case is soldered to the metal sheath of the cable, as is the metal armor, screen or shielding. Such a solder connection is difficult to achieve in the field and requires a high level of skill to be successfully completed.
Another approach taken in the art to connect a metal bridging element to the metal armor, screen or shielding of a power cable is the provision of two metal rings which encircle the cable and can be fastened together, for example, by bolts or screws. The ends of the armor and metal element are placed between the two rings which are then fastened together. A metal braid can be soldered to the lead or aluminum sheath of the cable, if present, and positioned between the rings to provide electrical connection to such sheath. This electrically and mechanically connects the armor, screen or shielding and the metal bridging element. Such a system is generally used with polymer insulated electrical cables used at low voltages, i.e. below one thousand volts. Its use has been limited to polymer insulated cables because of their relative insensitivity to moisture ingress. The use of the metal rings to clamp the armor and the metal bridging element does not provide a seal to prevent water from wicking along the armor and entering the joint area, a situation which could be detrimental to paper insulated cables.
In U.K. Patent Specification No. 2,005,091 to BICC Limited a method of terminating wire-armored lead sheathed paper insulated power cables is disclosed. The method uses a complex gland member for clamping the armor wires of the cable. The gland provides only one clamping area and thus is generally used for terminating armored power cables. It would be unsuitable for use to connect the armor of power cables at a joint between cables without additional complex components. The gland is electrically connected to the lead sheath of the power cable by use of a composite heat-recoverable tube having an inner sleeve of metal which softens at the recovery temperature of an outer tube. The gland is of complex structure and is difficult to install in the field.